Switching mode power supplies with primary side regulation are widely used in cell phone chargers, adapters for handheld electronics and auxiliary power supplies because of its good features. The key of the primary side regulation is output voltage and load condition can be sensed by accurate sampling a voltage across an auxiliary winding. However, for conventional primary side regulation, the output voltage and the load condition can be detected only during the conduction period of the secondary rectifying diode. This feature results in unqualified dynamic load response, especially when the switching mode power supply is operating with lower switching frequency.
FIG. 1 illustrates several working waveforms of a prior art switching mode power supply which operates in discontinuous conduction mode. As shown in FIG. 1, wherein Ip represents a current flowing through a primary switch, IS represents a current flowing through a secondary rectifying diode, VO represents an output voltage provided to a load, IO represents a load current, and Vaux represents a voltage across an auxiliary winding. Generally, the operation of the switching mode power supply are described as follows:
(1) During the time period D1, the primary switch is turned ON, the current IP flowing through the primary switch increases linearly from zero to a peak value Ilim. The energy is drawn from the input and stored in a primary winding of a transformer.
(2) During the time period D2, the primary switch is turned OFF, the energy stored in the primary winding forces the secondary rectifying diode to be turned ON. The output voltage and the forward voltage of the secondary rectifying diode are reflected to the auxiliary winding. At the end of the time period D2, all the energy stored in the primary winding has been delivered to the output.
(3) During the time period D3, the current IS flowing through the secondary rectifying diode diminishes to zero, the energy stored in the transformer also reaches zero. One skilled in the relevant art will recognize that “the time period D3” is the discontinuous conduction time period of the switching mode power supply.
As shown in FIG. 1, when the load current IO increases during the time period D3, the output voltage VO decreases accordingly. However, the voltage Vaux across the auxiliary winding can not reflect the output voltage and the load condition during this time period D3. The output voltage and the load condition can NOT be detected until the time period D2 of the next switching cycle. This leads to a large undershoot at the output voltage since the time period D3 is too long especially in light load or no load conditions. The overall system efficiency is reduced.